aPTT in Heparinized Patients: Influence of the Interval Between Sampling and Testing

Activated partial thromboplastin times (aPTTs) from 29 whole blood samples were drawn from patients receiv ing unfractionated heparin through a constant intravenous drip. Three aPTTs were determined for each whole blood sample. The first aPTT was performed on a separated portion of the original sample at approximately 30 min from the time of sam pling. A second was performed on the same separated sample at approximately 90 min from the time of sampling, and the third was performed on the original sample at 90 min. The mean aPTT of the set of 30-minute separated samples was 79.7 seconds. The mean aPTT of the 90-minute separated samples was 72.8 seconds. The mean aPTT of the 90-minute original sample was 60.8 seconds. These results demonstrate that an aPTT performed 30 min after sampling yields a value signifi cantly greater than an aPTT performed at 90 min. The lower aPTT values may lead to an unnecessary increase in heparin dosage. This situation underscores the need to further investi gate the aPTT as a foundation for the standardization of the protocol for the use of heparin.